I don't know when or where it was invented, but somehow Sudoku has come from nowhere to become one of the most popular ways to while away a bus journey. What's more, you don't even need a puzzle book to play it because there are so many versions of the game available for mobile phones.

Astraware's take on the format is very impressive and is actually one of the most well-executed I've seen anywhere. Obviously the scenario is the same as always - you have a 9 x 9 grid with a series of nine boxes and nine cells. You need to fill up all the cells with the numbers one to nine without repeating a number in any row, column or box.

But what's great about this game is the loving attention that's been given to its presentation. When you first start out, you're given an animated introduction to Sudoku, explaining all the rules in a succinct manner. You can then choose to start a new puzzle from the default ones built into the game, create your own puzzle, or download one from the Astraware site (you'll need to register for the full version to do this).

There are six difficulty levels to choose from, ranging from 'Beginner' to 'Diabolical,' which lets you develop your skills at your own pace. The in-game options are just as impressive. You can add pencilmarks to assist you, get hints, change the display and set the level of assistance.

Graphically, the game is tastefully presented and the board is very clear. There's even a clock and a battery indicator at the top to remind you not to lose track of things in the real world!

All in all, one of the best phone-based Sudoku games you'll find.